diff --git a/dom/media/webm/WebMBufferedParser.cpp b/dom/media/webm/WebMBufferedParser.cpp
index 979308cf0c..c3f1266cca 100644
--- a/dom/media/webm/WebMBufferedParser.cpp
+++ b/dom/media/webm/WebMBufferedParser.cpp
@@ -425,6 +425,7 @@ void WebMBufferedState::NotifyDataArrived(const unsigned char* aBuffer, uint32_t
 }
 
 void WebMBufferedState::Reset() {
+  ReentrantMonitorAutoEnter mon(mReentrantMonitor);
   mRangeParsers.Clear();
   mTimeMapping.Clear();
 }